Britain’s most senior civil servants should no longer be asked to investigate accusations of ministerial misconduct because they do not have the “powers” to get to the truth, the former Cabinet Secretary, Lord Robin Butler, has suggested.

The investigation into the Andrew Mitchell “Plebgate” affair was scorned today as a “waste of time and resources” by John Tully, the chairman of the Metropolitan Police Federation, after it emerged that more than 800 officers will be questioned over the scandal.